6

I tighten the bolt super hard. Ride for a while. Then it comes loose again. I don't know if its my BB or the bolts. How can this be fixed??

2
  • 3
    What model or crank / bottom bracket are we dealing with?
    – alex
    Feb 11, 2013 at 7:10
  • Any chance of adding some pictures of the affected area?
    – freiheit
    Feb 11, 2013 at 18:12

2 Answers 2

11

By "non-drive" I assume you mean the left side. This is more apt to come loose than the right because of "precession" -- most crank bolts are right-hand thread on both sides, but the motion of the crank arm relative to the shaft tends to loosen the bolt on the left side, whereas it tends to tighten the bolt on the right side.

But if this is occurring it's because there IS motion between the crank arm and the shaft. This should not be, as the two should fit together so tightly as to prevent any motion. What has no doubt happened is that the bolt was installed too loosely at one point, the crank arm worked loose, and now the socket of the arm is misshapened so the two tend to move relative to each other all the time.

Using some Loctite (the "removable" variety) on the bolt will help temporarily, but ultimately something must be done about the misfit between the two parts. Basically either the arm must be replaced (and the shaft, if the situation goes on for too long), or (if this is a square shaft) you must carefully fashion a shim (perhaps out of aluminum pop can material) to create a better fit. (If it's a splined shaft you're probably SOL.) And regularly re-tighten the bolt -- don't wait for it to get loose.

3
  • Well, I don't know, I've heard that re-tightening it is a bad idea, as it can (over time) damage or even strip the threads. I'd remove the crank arms and use a torque wrench to screw them in to spec (usually around 40-50 Nm on a square taper.) Aug 19, 2013 at 9:44
  • 1
    @AleksandarDimitrov - How does retightening differ from removing and reinstalling?? The point is they must be kept tight, to prevent further damage. Riding with the arm loose WILL damage the shaft and the threads. Aug 19, 2013 at 11:12
  • Agreed that they have to be tight, otherwise damage will occur. Re-tightening to a higher-than-necessary torque I think I should've added. Re-installing comes with re-greasing the tapered shafts, which in general will help to make a more reliable connection. Aug 20, 2013 at 20:31
1

It’s well known that left-side crank arms come loose because of changing torque that causes play, as noted by Daniel. This is especially true on a street-ridden fixed gear because the torque REVERSES continually.

If this happens, you’re usually not SOL, because you can usually reef it harder. This sometimes/often is necessary a few times after installing the arm. It’s ALWAYS necessary on a fixed gear, on every install.

The solution is to ride it, tighten it, ride it, and tighten again. Repeating at least three times on a fixed gear.

BUT WATCH OUT, as this will cause the arm to sit deeper on the spindle every time!!! That means at a point the arm can rub on the bottom bracket, causing it to UNSCREW toward the arm! This will lock up your drivetrain if you are on a fixed gear, and that could cause a crash. I got lucky in that I was going slow when it happened to me!!!

The bolts rarely bust because it would be a liability for the manufacturer.

Low quality square-taper cranksets are inappropriate for street-ridden fixed gears, given this issue. I've had them go bad in as little as one day, even adhering to the best advice.

Not the answer you're looking for? Browse other questions tagged or ask your own question.